Zachary Freyberg

Roles of the Akt/GSK-3 and Wnt Signaling Pathways in Schizophrenia and Antipsychotic Drug Action

Dopamine D(2) receptor antagonism is a unifying property of all antipsychotic drugs in clinical use. Remarkably, the effector molecules through which these medications exert their actions remain poorly characterized. Increasing attention is being focused on Akt/glycogen synthase kinase-3 (GSK-3) and wingless (Wnt) signaling pathways, which have been associated with schizophrenia in a number of genetic and postmortem studies. Antipsychotic medications may treat symptoms of psychosis, at least in part, through modulation of levels and activity of Akt, GSK-3, and Wnt-related intracellular signaling. The authors review evidence that Akt/GSK-3 and Wnt-related pathways are involved in the pathogenesis of schizophrenia as well as details of intracellular events related to these molecules mediated by both typical and atypical antipsychotic medications. Further study of Akt/GSK-3 and Wnt signaling may ultimately lead to alternative therapeutics of schizophrenia-related disorders.

Signaling pathways in schizophrenia: emerging targets and therapeutic strategies

Dopamine D(2) receptor antagonism is a unifying property of all antipsychotic drugs in use for schizophrenia. While often effective at ameliorating psychosis, these drugs are largely ineffective at treating negative and cognitive symptoms. Increasing attention is being focused on the complex genetics of the illness and the signaling pathways implicated in its pathophysiology. We review targeted approaches for pharmacotherapy involving the glutamatergic, GABAergic and cholinergic pathways. We also describe several of the major genetic findings that identify signaling pathways representing potential targets for novel pharmacological intervention. These include genes in the 22q11 locus, DISC1, Neuregulin 1/ErbB4, and components of the Akt/GSK-3 pathway.

Treatment of depression in HIV positive individuals: A critical review

The primary goal of this paper is to provide a critical review of the literature on treatment of depression in HIV/AIDS. There is a substantial research literature documenting the efficacy of conventional antidepressants, particularly selective serotonin reuptake inhibitors (SSRIs), novel agents such as dehydroepiandrosterone, psychostimulants and some psychotherapies, particularly interpersonal and group psychotherapy for the treatment of depression in HIV. However, lack of comparative studies makes it difficult to draw a firm consensus regarding the best course of treatment. In devising a treatment plan, clinicians should take into account stage of HIV illness, co-morbid illnesses such as Hepatitis B and C, the potential for drug interactions with antiretroviral and other medications used to treat HIV and patient preference.

The membrane raft protein Flotillin-1 is essential in dopamine neurons for amphetamine-induced behavior in Drosophila

The dopamine transporter (DAT) is the primary molecular target responsible for the rewarding properties of the psychostimulants amphetamine (AMPH) and cocaine. AMPH increases extracellular dopamine (DA) by promoting its nonexocytotic release via DAT-mediated efflux. Previous studies in heterologous cells have shown that phosphorylation of the amino terminus of DAT is required for AMPH-induced DA efflux but not for DA uptake. However, the identity of many of the modulatory proteins and the molecular mechanisms that coordinate efflux and the ensuing behavioral effects remain poorly defined. Here, we establish a robust assay for AMPH-induced hyperlocomotion in Drosophila melanogaster larvae. Using a variety of genetic and pharmacological approaches, we demonstrate that this behavioral response is dependent on DA and on DAT and its phosphorylation. We also show that methylphenidate (MPH), which competitively inhibits DA uptake but does not induce DAT-mediated DA efflux, also leads to DAT-dependent hyperlocomotion, but this response is independent of DAT phosphorylation. Moreover, we demonstrate that the membrane raft protein Flotillin-1 is required for AMPH-induced, but not MPH-induced, hyperlocomotion. These results are the first evidence of a role for a raft protein in an AMPH-mediated behavior. Thus, using our assay we are able to translate molecular and cellular findings to a behavioral level and to differentiate in vivo the distinct mechanisms of two psychostimulants.

Molecular pathophysiology of metabolic effects of antipsychotic medications

Antipsychotic medications are associated with major metabolic changes that contribute to medical morbidity and a significantly shortened life span. The mechanisms for these changes provide us with a broader understanding of central nervous and peripheral organ-mediated metabolic regulation. This paper reviews an extensive literature regarding putative mechanisms for effects of antipsychotic medications on weight regulation and glucose homeostasis as well as potential inherent metabolic risks of schizophrenia itself. We present a model suggesting that peripheral antipsychotic targets play a critical role in drug-induced weight gain and diabetes. We propose that a better understanding of these mechanisms will be crucial to developing improved treatments for serious mental illnesses as well as providing potentially novel therapeutic targets of metabolic disorders including diabetes.

Mechanisms of amphetamine action illuminated through optical monitoring of dopamine synaptic vesicles in Drosophila brain

Amphetamines elevate extracellular dopamine, but the underlying mechanisms remain uncertain. Here we show in rodents that acute pharmacological inhibition of the vesicular monoamine transporter (VMAT) blocks amphetamine-induced locomotion and self-administration without impacting cocaine-induced behaviours. To study VMATs role in mediating amphetamine action in dopamine neurons, we have used novel genetic, pharmacological and optical approaches in Drosophila melanogaster. In an ex vivo whole-brain preparation, fluorescent reporters of vesicular cargo and of vesicular pH reveal that amphetamine redistributes vesicle contents and diminishes the vesicle pH-gradient responsible for dopamine uptake and retention. This amphetamine-induced deacidification requires VMAT function and results from net H+ antiport by VMAT out of the vesicle lumen coupled to inward amphetamine transport. Amphetamine-induced vesicle deacidification also requires functional dopamine transporter (DAT) at the plasma membrane. Thus, we find that at pharmacologically relevant concentrations, amphetamines must be actively transported by DAT and VMAT in tandem to produce psychostimulant effects.

Neuropsychiatric aspects of infectious diseases.

Among the critically ill, infectious diseases can play a significant role in the etiology of neuropsychiatric disturbances. All critical care physicians are familiar with delirium as a secondary complication of systemic infection. This article focuses on key infectious diseases that commonly and directly produce neuropsychiatric symptoms, including direct infection of the central nervous system.

Increased localization of APP-C99 in mitochondria-associated ER membranes causes mitochondrial dysfunction in Alzheimer disease

In the amyloidogenic pathway associated with Alzheimer disease (AD), the amyloid precursor protein (APP) is cleaved by β‐secretase to generate a 99‐aa C‐terminal fragment (C99) that is then cleaved by γ‐secretase to generate the β‐amyloid (Aβ) found in senile plaques. In previous reports, we and others have shown that γ‐secretase activity is enriched in mitochondria‐associated endoplasmic reticulum (ER) membranes (MAM) and that ER–mitochondrial connectivity and MAM function are upregulated in AD. We now show that C99, in addition to its localization in endosomes, can also be found in MAM, where it is normally processed rapidly by γ‐secretase. In cell models of AD, however, the concentration of unprocessed C99 increases in MAM regions, resulting in elevated sphingolipid turnover and an altered lipid composition of both MAM and mitochondrial membranes. In turn, this change in mitochondrial membrane composition interferes with the proper assembly and activity of mitochondrial respiratory supercomplexes, thereby likely contributing to the bioenergetic defects characteristic of AD.

Structural Basis of Dopamine Receptor Activation

G protein-coupled receptors (GPCRs) are seven transmembrane (TM) proteins representing the largest and most universally expressed cell surface receptors and are present in almost all species and in a wide variety of cells. Here we will focus our attention on the catecholamine-binding GPCRs and in particular on the dopamine receptors. The catecholamine-binding GPCRs form a group of rhodopsin-like GPCRs composed of adrenoceptors, which are endogenously activated by epinephrine and norepinephrine, and dopamine receptors. We review the different “molecular switches” involved in GPCR activation and we emphasize the importance of extracellular loop 2 (ECL2) in ligand binding. A better understanding of the functional role of ECL2 can be achieved after the release of the crystal structures of B2AR and rhodopsin, which are consistent with dopamine D2 receptor substituted cysteine accessibility method (SCAM) experimental data. Even though reconstituted GPCR monomers appear sufficient to activate a G protein, in the native setting their dimerization/oligomerization may modulate activation through changes at the dimerization interface or a larger-scale reorientation of the protomers. Therefore, the structural aspects of oligomerization and their importance for receptor activation and signaling are also addressed.

Role for VGLUT2 in selective vulnerability of midbrain dopamine neurons

Parkinson’s disease is characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc). DA neurons in the ventral tegmental area are more resistant to this degeneration than those in the SNc, though the mechanisms for selective resistance or vulnerability remain poorly understood. A key to elucidating these processes may lie within the subset of DA neurons that corelease glutamate and express the vesicular glutamate transporter VGLUT2. Here, we addressed the potential relationship between VGLUT expression and DA neuronal vulnerability by overexpressing VGLUT in DA neurons of flies and mice. In Drosophila, VGLUT overexpression led to loss of select DA neuron populations. Similarly, expression of VGLUT2 specifically in murine SNc DA neurons led to neuronal loss and Parkinsonian behaviors. Other neuronal cell types showed no such sensitivity, suggesting that DA neurons are distinctively vulnerable to VGLUT2 expression. Additionally, most DA neurons expressed VGLUT2 during development, and coexpression of VGLUT2 with DA markers increased following injury in the adult. Finally, conditional deletion of VGLUT2 made DA neurons more susceptible to Parkinsonian neurotoxins. These data suggest that the balance of VGLUT2 expression is a crucial determinant of DA neuron survival. Ultimately, manipulation of this VGLUT2-dependent process may represent an avenue for therapeutic development.